Fuel cells were invented in 1839 by Sir William Grove. A fuel cell is an electrochemical device which directly combines a fuel and an oxidant such as hydrogen and oxygen to produce electricity and water. It has an anode and a cathode separated by an electrolyte. Hydrogen is oxidized to hydrated protons on the anode with an accompanying release of electrons. At the anode, oxygen reacts with protons to form water, consuming electrons in the process. Electrons flow from the anode to the cathode through an external load, and the circuit is completed by ionic current transport through the electrolyte.
Fuel cells do not pollute the environment. They operate quietly, and high temperature fuel cells have a potential efficiency of ca. 80 percent. Virtually any natural or synthetic fuel from which hydrogen can be extracted--by steam reforming, for example--can be employed.
As suggested above, the anode is a major component of a high temperature fuel cell, and a successful fuel cell of that character which will remain stable when heated at high temperatures for long periods of time--for example, at the 500-750.degree. C. operating temperatures and in the corrosive environment of molten carbonate fuel cells.
Anodes for high temperature fuel cells are typically based on nickel although other metals have been proposed. These are principally cobalt (which is of limited practicality because of its posture as a strategic metal) and copper (see U.S. Pat. No. 4,752,500 issued Jun. 21, 1988 to Donado for PROCESS FOR PRODUCING STABILIZED MOLTEN CARBONATE FUEL CELL POROUS ANODES).
The base metals suitable for high temperature fuel cell anodes undergo sintering at the temperatures to which the anodes of high temperature fuel cells are subjected. As a result, the anode, which is required to be porous, consolidates with use; and the pores close, causing countless problems. This consolidation occurs because of the reducing atmosphere, compressive load, and temperatures utilized in molten carbonate and other high temperature fuel cell processes. Furthermore, the stack of anodes in the cell bundle gets thinner and thinner with use. This results in pore closure which causes gaps in the stack and with resulting lapses in electronic and ionic communication through the stack and a consequent reduction in operating efficiency.
Other potential microstructural instability mechanisms in a porous anode under cell operating conditions are:
Compressive Creep--Particles are drawn closer together as the metal yields under load.
Particle Rearrangement--Large pores collapse as the particles rearrange via grain boundary sliding under compressive load.
Additives have been employed in an effort to stabilize nickel-based anodes intended for high temperature fuel cells. One commonly suggested stabilizing additive for nickel in fuel cell anodes is chromium.
Currently state-of-the-art are Ni-Cr anodes fabricated by: blending Ni and Cr powders, forming the mixture into a sheet form, and sintering the sheet into a coherent, porous, green structure. The green structure is installed directly in the fuel cell in which the anode is to be used; the fuel cell is brought to operating temperature; and fuel and oxidant are introduced into the cell. The Cr constituent of the anode thereupon oxidizes and forms Cr.sub.2 O.sub.3 and LiCrO.sub.2 on the surface of the nickel base metal.
External oxides have only a limited ability to inhibit anode creep. Consequently creep of unacceptable magnitude is a continuing problem, even in state-of-the-art Ni-Cr anodes.
Nickel-aluminum fell cell anodes in which the aluminum is employed to stabilize the nickel base metal have also been proposed by a number of investigators, but their efforts to date have met with only limited success.
In any event, it has been found that internal oxidation of the alloying element (Cr or Al in nickel) is necessary to obtain a creep resistant anode. Internal oxidation of a fuel cell anode is a prolonged process because it involves solid state diffusion of oxygen in nickel at a controlled temperature and atmosphere. For example, in one investigation where the anode composition was Ni+Cr, the internal oxidation step was carried by heating the anode structure at 600.degree.-800.degree. C. for 24 hours in an atmosphere containing 80-100 volumes of steam to one volume hydrogen.
Also, to obtain a structure with the requisite porosity, Ni-Al fuel cell anodes are generally prepared by sintering a mixture of nickel and aluminum powders. However, nickel and aluminum powder mixtures do not readily form alloys when heated because Al oxidizes in normal reducing atmospheres employed in sintering processes. Therefore, it is difficult a to fabricate Ni-Al anode from Ni and Al powders. Moreover, if one starts with a pre-alloyed Ni-Al powder, sintering of the powder into a coherent structure is also difficult in normal reducing atmospheres because the gas usually contains a residual oxygen that readily oxidizes the Al phase, and the oxide product inhibits powder sintering. The problem described in the preceding paragraph has resulted in the development of a multi-step process designed to produce a usable Ni-Al fuel cell anode. In that process, Ni-Al powder is oxidized under controlled conditions such that the surfaces of the resulting particles are NiO powder and the interiors consists of aluminum oxide and metallic nickel. By then heat treating the oxidized powder in a reducing atmosphere, the NiO is reduced; and the particles are sintered together.
This process is undesirably complex. And it is expensive and time-consuming. For example, in an investigation where the anode composition was Ni+Al, the oxidation step was performed in an oxygen-containing atmosphere at 700.degree.-1000.degree. C. for 1-10 hours to oxidize both the Ni and Al. Afterwards, the Ni phase had to be reduced back to metal in a hydrogen-containing atmosphere at 600.degree.-1000.degree. C. for 0.5-2.0 hours.